A concept of negative interstitial pressure based on pressures in implanted perforated capsules.

• Interstitial pressure has usually been measured by inserting a very small needle into a tissue and then determining the minimum pressure required to make fluid flow from the needle into the tissue." Unfortunately, even when extremely small needles are used for this purpose, such as 25 to 30 gauge, the external diameter of the needle is still some 300 to 500 times as large as the widths of the tissue spaces. The insertion of such needles into the tissues must distort the spaces greatly and could easily give a false measure of the interstitial pressure. For this reason, we attempted to find another method for measuring interstitial pressure and developed a technique using perforated capsules implanted in the tissues. After such capsules have remained in place for two to three weeks they become filled with interstitial fluid, and the intracapsular pressure can be measured by inserting a minute needle through the skin and then through one of the capsule perforations into the inner cavity. During the last two years we have implanted approximately 200 such capsules in different tissue spaces of dogs. Contrary to general belief regarding interstitial pressure, we have found that the pressure inside these capsules is normally negative rather than positive. On the other hand, in edematous tissues, the measured pressures have always been positive." This paper presents a series of different experiments performed to determine whether the pressure measured inside the cap-